[clang][bytecode] Don't diagnose const assignments... (#192593)

... when we're in CPCE mode.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -579,9 +579,11 @@ bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   if (llvm::is_contained(S.InitializingBlocks, Ptr.block()))
     return true;
 
-  const QualType Ty = Ptr.getType();
-  const SourceInfo &Loc = S.Current->getSource(OpPC);
-  S.FFDiag(Loc, diag::note_constexpr_modify_const_type) << Ty;
+  if (!S.checkingPotentialConstantExpression()) {
+    const QualType Ty = Ptr.getType();
+    const SourceInfo &Loc = S.Current->getSource(OpPC);
+    S.FFDiag(Loc, diag::note_constexpr_modify_const_type) << Ty;
+  }
   return false;
 }
 

clang/test/AST/ByteCode/cxx20.cpp

@@ -1345,3 +1345,12 @@ namespace ExpandOnOPTEPointers {
   }
   static_assert(test());
 }
+
+namespace ConstIntPotentialConstantExpr {
+  /// NO error about a constexpr function that's never a constant expression.
+  constexpr int Const() {
+    const int a = 10; // both-note {{declared const here}}
+    a = 20; // both-error {{cannot assign to variable 'a' with const-qualified type 'const int'}}
+    return 1;
+  }
+}